manager.c: Generate valid XML if attribute names have leading digits.

The XML Manager Event Interface (amxml) now generates attribute names
that are compliant with the XML 1.1 specification. Previously, an
attribute name that started with a digit would be rendered as-is, even
though attribute names must not begin with a digit. We now prefix
attribute names that start with a digit with an underscore ('_') to
prevent XML validation failures.

This is not backwards compatible but my assumption is that compliant
XML parsers would already have been complaining about this.

ASTERISK-29886 #close

Change-Id: Icfaa56a131a082d803e9b7db5093806d455a0523